How can a I read and update the keyframe value of a material node in Blender 2.8 via Python?
I don't know if this is the best way to do it, but it does exactly what I want it to do.

I have a Group of nodes. As part of this Group, there is a Value input node. I want to keyframe the default value of this Value node. So this is how it works.

Inserting a new keyframe point

1- Get a reference to the Group node:

group = bpy.data.materials['Material'].node_tree.nodes['Group']


2- Get a reference to the Value node within the Group:

node = group.node_tree.nodes['ValueNodeName']


3- Set the default value of the output of the Value node, and insert a keyframe (in this example, the value to insert is 1, and the frame number is 21)

node.outputs['Value'].default_value = 1
node.outputs['Value'].keyframe_insert('default_value', frame = 21)


Repeat step 3 for each new keyframe point to insert.

Overwriting an existing keyframe point

If I needed to overwrite an existing keyframe point, I could enter another keyframe point for the very same frame number. For example, if I wanted to update the keyframe point at frame 21 from value 1 to value 33:

node.outputs['Value'].default_value = 33
node.outputs['Value'].keyframe_insert('default_value', frame = 21)


Accessing a Keyframe Point from an F-Curve

Instead of adding, deleting and overwriting keyframe points from the node's output properties, we could get a reference to the F-Curve used to animate those properties and modify them that way. If material is a reference to the material containing our value node, this is how we could proceed:

action = material.node_tree.animation_data.action
if action:
    fc = action.fcurves.find(data_path)
        if fc:
            for kfp in fc:
                print(kfp.co)


The previous snippet would print the coordinates of all the keyframe points belonging to the F-Curve stored under data path data_path. For example, if the name of our value node were MyValueNode, the data path would be 

data_path = 'nodes['MyValueNode'].outputs[0].default_value'